[Qgis-user] inconsistenty when calculating area depending on file type or projection?

Tue Oct 18 09:22:01 PDT 2016

It was my fault. I was used to consider plane areas as "correct",
forgetting ellipsoidal areas. So, QGIS give us freedom to calculate planar
or ellipsoidal (geodetical) areas, but we must be careful about settings;
Is not it?. Sometimes, too much freedom is annoying!

But, How can I calculate UTM (planar) area from a GEO layer in one step?

Thank you very much for your time and so sorry if I'm doing basic questions.

Carlos

2016-10-16 17:59 GMT-05:00 Nyall Dawson <nyall.dawson at gmail.com>:

> On 16 October 2016 at 05:20, Carlos Cerdán <sig.upagu at gmail.com> wrote:
> > Hi Nyall (and list)
> >
> > A small projecto with UTM and GEO layers can be downloaded from dropbox:
> >
> > https://www.dropbox.com/s/c4cfb48r5326vrk/Cajamarca.zip?dl=0
> >
> > Attribute table has the correct area in AreaKm2 field (and in these
> > unities). "AreaOTFoff" is the area given by field calculator when "On The
> > Fly transformation" is deactivated. "AreaOTF_on" is the area with it
> > activated.
> >
> > So: when I have a GEO layer, I can't calculate correct area directly in
> that
> > layer: I've to reproject to UTM and deactivate automatic transformation
> of
> > SRCs, to use field calculator expecting correct values.
> >
> > OS : Ubuntu 16.04
> > QGIS : 2.16.3
> > SRC settings: by default (ellipsoide WGS84 for measures)
>
> Thanks for sending this through, but again - I can't find any issue here.
>
> The different area calculations come from the difference between
> measuring the area using a flat plane (OTF off, no ellipsoid set) vs
> measuring the area using an ellipsoid (OTF on, ellipsoid set under the
> project "general" settings). When OTF is on QGIS calculates the same
> area regardless of whether the geographic or projected source layer is
> used.
>
> Hope that clarifies!
>
> Nyall
>
>
>
> >
> > Thank you for your interest
> >
> > Carlos Cerdán
> >
> >
> > 2016-10-14 15:14 GMT-05:00 Nyall Dawson <nyall.dawson at gmail.com>:
> >>
> >> On 15 Oct 2016 12:51 AM, "Carlos Cerdán" <sig.upagu at gmail.com> wrote:
> >> >
> >> > Hi Nyall
> >> >
> >> > I'm afraid that 2.16 has still this issue. I've loaded an UTM-17 south
> >> > layer (my zone) and a Lat-long layer and:
> >> >
> >> > 1. SRC was seted in UTM
> >> >
> >> > 2. In UTM layer, if OTF SRC transformation is active, I get different
> >> > area than if it's deactivated. Correct value is the last one.
> >> >
> >> > 3. In Lat-long layer, if OTF SRC transformation is active, calculated
> >> > area is same as the wrong value of first layer. I can't get the
> correct
> >> > value in this layer, so I have to reproject into a new one and do
> step 2
> >> > (with OTF deactivated).
> >> >
> >> > What about a general option to set the prefered SRC to calculate areas
> >> > and lengths with OTF active?
> >>
> >> Hi Carlos,
> >>
> >> Can you please share your file? Cut it down to just a few polygons and
> let
> >> me know what area you expect to see. Email direct to myself.
> >>
> >> Thanks!
> >>
> >> Nyall
> >>
> >>
> >> >
> >> > Regards from Peru
> >> >
> >> > Carlos
> >> >
> >> >
> >> >
> >> > 2016-10-12 18:06 GMT-05:00 Nyall Dawson <nyall.dawson at gmail.com>:
> >> >>
> >> >> On 12 Oct 2016 11:56 PM, "Carlos Cerdán" <sig.upagu at gmail.com>
> wrote:
> >> >> >
> >> >> > AFAIK, It also is needed to turn off "on the fly SRC
> transformation"
> >> >> > to get correct area values.... Or QGIS has fixed this point?
> >> >>
> >> >> Everything should be fixed in recent versions, and I very (VERY) much
> >> >> want to know if any issues are still encountered.
> >> >>
> >> >> Calculating area/length is a core task for a GIS and we need to make
> >> >> sure it's rock solid. (Which it should be since 2.16!)
> >> >>
> >> >> Nyall
> >> >>
> >> >>
> >> >> >
> >> >> > If you can't get correct area values, check out about it....
> >> >> >
> >> >> >
> >> >> > 2016-10-12 7:40 GMT-05:00 DelazJ <delazj at gmail.com>:
> >> >> >>
> >> >> >> Hi,
> >> >> >> To complete Nicolas answer, you should check what are the
> >> >> >> measurements options set in Project --> Project Properties -->
> General tab.
> >> >> >> See also
> >> >> >> http://docs.qgis.org/2.14/en/docs/user_manual/introduction/
> general_tools.html#measuring
> >> >> >>
> >> >> >> 2016-10-12 14:07 GMT+02:00 Nicolas Cadieux
> >> >> >> <nicolas.cadieux at archeotec.ca>:
> >> >> >>>
> >> >> >>> Hi,
> >> >> >>> You may be calculating square degrees and not metres.  It can
> >> >> >>> depend on the crs depending on the tools you are using.
> >> >> >>> Nicolas
> >> >> >>>
> >> >> >>> Le 11 oct. 2016 à 08:54, Martina Schäfer [via OSGeo.org] <[hidden
> >> >> >>> email]> a écrit :
> >> >> >>>
> >> >> >>>> I experienced some confusion with calculation of area using the
> >> >> >>>> field calculator in QGIS version 2.16.3. Since I'm using MapInfo
> >> >> >>>> Professional as well, I mainly use tab-files that I can open in
> both
> >> >> >>>> programmes, but occasionally I save as shapefile since this
> used to be the
> >> >> >>>> default in QGIS.
> >> >> >>>>
> >> >> >>>> When comparing files, I coincidently realized that there was a
> >> >> >>>> mismatch in calculated area for the shapefile and the tab-file
> for exactly
> >> >> >>>> the same polygons! I used the field calculator in the attribute
> table in
> >> >> >>>> both cases, but for the shapefile the resulting areas were
> almost doubled in
> >> >> >>>> area compared to the tab-file. Any idea why this is happening?
> >> >> >>>>
> >> >> >>>> I also realized similar differences when calculating area in a
> >> >> >>>> file where projection has been converted from SWEREF99TM (a
> Swedish national
> >> >> >>>> projection) to WGS84. There differences occurred in both the
> tab and
> >> >> >>>> shapefile compared to the area calculated for the same tab-file
> in MapInfo.
> >> >> >>>> Again I find this very confusing!
> >> >> >>>>
> >> >> >>>> I need to rely on the area-calculations thus I really hope
> someone
> >> >> >>>> here can explain to me what is happening!
> >> >> >>>>
> >> >> >>>> Thanks in advance,
> >> >> >>>> Martina
> >> >> >>>>
> >> >> >>>>
> >> >> >>>>
> >> >> >>>>
> >> >> >>>> ________________________________
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> >> >> >>>> discussion below:
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> calculating-area-depending-on-file-type-or-projection-tp5290228.html
> >> >> >>>> To start a new topic under Quantum GIS - User, email [hidden
> >> >> >>>> email]
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> >> >> >>>> NAML
> >> >> >>>
> >> >> >>>
> >> >> >>> ________________________________
> >> >> >>> View this message in context: Re: inconsistenty when calculating
> >> >> >>> area depending on file type or projection?
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> >> >> >>> Nabble.com.
> >> >> >>>
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